Receptor-interacting serine/threonine-protein kinase 2 (RIPK2, also called RICK, RIP2, CARDIAK, and CARD3) has been implicated in a variety of functions including: integrating signals for innate and adaptive immune systems, regulating apoptosis, controlling a myogenic differentiation checkpoint, and regulating nuclear-factor-kappa-beta (NFkB) and Jun N-terminal kinase (JNK) activation. RIPK2 is composed of an N-terminal serine/threonine kinase catalytic domain and a C-terminal region containing a caspase activation and recruitment domain (CARD).
RIPK2 physically interacts with CLARP, a caspase-like molecule known to bind to Fas-associated protein with death domain (FADD) and caspase-8. Expression of RIPK2 promoted the activation of caspase-8 and potentiated apoptosis induced by Fas ligand, FADD, CLARP, and caspase-8. Deletion mutant analysis revealed that both the kinase domain and caspase-recruitment domain were required for RIPK2 to promote apoptosis. Significantly, expression of a RIPK2 mutant in which the lysine of the putative ATP-binding site at position 38 was replaced by a methionine functioned as an inhibitor of CD95-mediated apoptosis. Thus, RIPK2 represents a novel kinase that may regulate apoptosis induced by the CD95/Fas receptor pathway.
Because expression of RIPK2 affects the regulation of apoptosis in a variety of cell types, RIPK2 activity may be an important factor in the development of disease states in which regulation of apoptosis is critical. Significantly, RIPK2 protein level is increased in the frontal cortex of patients with Alzheimer's disease (Engidawork et. al., 2001, Biochem. Biophys. Res. Commun. 281: 84-93).
Analysis of RIPK2 deficient mice indicates that RIPK2 is required for regulation of innate and adaptive immune and inflammatory responses. RIPK2 deficient mice were born in the expected Mendelian ratio, and showed no gross developmental abnormalities or abnormal composition of lymphocytes as determined by flow cytometry (Kobayashi et. al., 2002, Nature 416: 194-199; Chin et. al., 2002, Nature 416: 190-194). However, these mice exhibited a decreased ability to defend against infection by the intracellular pathogen Listeria monocytogenes (Chin et. al., 2002). RIPK2 deficient macrophages and T-cells showed severely reduced NFkB activation (Kobayashi et. al., 2002; Chin et. al., 2002). RIPK2 deficiency also resulted in impaired interferon-.gamma. production in both T.sub.H1 and natural killer cells and impaired T.sub.H1-cell differentiation (Kobayashi et. al., 2002; Chin et. al., 2002). Analysis of RIPK2 deficient mice suggests that RIPK2 is a candidate target for immune intervention.
RIPK2 has been reported to physically associate with several proteins involved in receptor mediated signaling through the tumor necrosis factor (TNF) family of receptors including TNFR-1, TNFR-2, Fas (CD-95/APO-1), lyphotoxin-.beta. receptor, CD40, CD30, OX-40, DR3, DR4, and DR5. For example, RIPK2 physically interacts with CLARP, a caspase-related protein that interacts with Caspase-8 and FADD (a protein which associates with the Fas/CD-95 and TNFR-1 receptors) (Inohara et. al., 1998). CLARP could therefore function as an adapter molecule to link RIPK2 to proximal components of the receptor signaling complex.
RIPK2 also physically interacts with Caspase-1 (Thome et. al., 1998; Humke et. al, 2000, Cell 103: 99-111). This protein interaction is mediated by CARD domains in the C-terminus of RIPK2 and in the prodomain of Caspase-1 (Thome et. al., 1998; Humke et. al., 2000). RIPK2 enhances the activation of Caspase-1 by promoting its oligomerization which leads to processing of adjacent pro-Caspase-1 protein (Humke et. al., 2000). The association between RIPK2 and Caspase-1 can be abrogated by the ICEBERG protein, which inhibits and/or displaces RIPK2 by binding Caspase-1 through its own CARD domain. (Humke et. al., 2000).
RIPK2 has been reported to associate directly with p75 receptor in a nerve growth factor (NGF) dependent fashion (Khursigara et. al., 2001) and with several receptor associating proteins including TRAF1, TRAF2, TRAF5, and TRAF6 (Thome et. al., 1998; McCarthy et. al., 1998). Co-expression of CD40 receptor, RIPK2, TRAF1 and TRAF2 resulted in association of RIPK2 with CD40 (McCarthy et. al., 1998). Likewise, co-expression of TNFR-1 receptor, RIPK2, TRADD, TRAF1 and TRAF2 resulted in association of RIPK2 with TNFR-1 (McCarthy et. al., 1998). Collectively, these data suggest that RIPK2 is a component of the p75, CD40, Fas/CD-95 and TNFR-1 receptor signaling complexes.
RIPK2 activity appears to be altered by interaction with ligands. For example, expression of polypeptides comprising CARD domains with high affinity for RIPK2 protein binding partners may prevent RIPK2 from physically associating with other CARD domain containing proteins (Humke et. al., 2000). Protein-protein interactions mediated by CARD domains have also been reported to be disrupted by nitric oxide (NO) (Zech et. al., 2003, Biochem J. 371(Part 3): 1055-64). Compounds that alter the serine-kinase activity of RIPK2 may also influence RIPK2 function. Methods for assessing the kinase activity of RIPK2 have been described (Inohara et. al., 1998; Thome et. al., 1998; McCarthy et. al., 1998; Navas et. al., 1999). Methods for screening for compounds that modulate serine-threonine kinase activity have been disclosed (US2003/0134310A1; WO 02/14542). In addition, anti-sense oligonucleotides designed to inhibit RIPK2 have been described (U.S. Pat. No. 6,426,221 B1).
Because of the multiple therapeutic values of compounds targeting receptor mediated signaling pathways that modulate apoptosis, cellular differentiation, and immune response, and the essential regulatory role played by RIPK2, there is a need in the art for novel compounds that can inhibit RIPK2.